Hitherto, a working machine in which a part of a driving mechanism is designed to be electrically operated has been proposed. Such a working machine includes a hydraulic pump which hydraulically drives movable portions, for example, a boom, an arm, and a bucket, and an AC motor (an electrical generator) is connected to an internal combustion engine (an engine) driving the hydraulic pump to assist a driving force of the engine and to return electricity obtained by the generation of electricity to a DC bus (a DC busbar) via an inverter.
Furthermore, the working machine includes, for example, a working component such as an upper rotation body of a construction machine in many cases. In such a case, the working machine includes a working motor assisting the hydraulic motor in addition to the hydraulic motor driving the working component. For example, when the upper rotation body is rotated, the driving of the hydraulic motor is assisted by the AC motor while the rotation is accelerated, a regenerative operation is performed in the AC motor while the rotation is decelerated, and the generated electricity is returned to the DC bus via the inverter.
A storage battery (a battery) is connected to the DC bus via a converter, and the battery is charged by the electricity generated by the AC motor. Alternatively, the electricity is received and transmitted between the DC bus and the AC motor connected thereto.
In the working machine, the voltage of the DC bus is set to be as high as, for example, several hundred volts in order to drive a large working component, but it is desirable to decrease the voltage of the DC bus for the operator's safety during the maintenance thereof. For example, in the device disclosed in PTL 1, the voltage of the DC bus is consumed by resistance in a manner such that a resistor and a switch are connected in series to each other between the positive and negative interconnections of the DC bus.